From muguet@poly.polytechnique.fr Thu Jun 27 08:56:48 1996 Received: from polytechnique.polytechnique.fr for muguet@poly.polytechnique.fr by www.ccl.net (8.7.5/950822.1) id IAA01227; Thu, 27 Jun 1996 08:26:32 -0400 (EDT) Received: from poly.polytechnique.fr (tCehI2gji/111H6tNmW/wkdyDYpm6aMD@poly.polytechnique.fr [129.104.252.1]) by polytechnique.polytechnique.fr (8.6.12/8.6.12) with ESMTP id OAA18272 for ; Thu, 27 Jun 1996 14:29:19 +0200 Received: (from muguet@localhost) by poly.polytechnique.fr (8.6.10/8.6.6) id OAA06598 for CHEMISTRY@www.ccl.net; Thu, 27 Jun 1996 14:27:06 +0200 Date: Thu, 27 Jun 1996 14:27:06 +0200 From: muguet francis Message-Id: <199606271227.OAA06598@poly.polytechnique.fr> To: CHEMISTRY@www.ccl.net Subject: MCSCF freq. + scaling factors Dear CCL Netters CHEMISTRY@www.ccl.net Ref: CAS vs HF frequencies, vibration scaling factors, I would like to add my grain of salt to these 2 threads. 1/ I would tend to think that MCSCF ( and then CAS ) frequencie are much better than HF and ALSO MP2. In the case, where static correlation is important, no doubt MCSCF is better. It has been argued (in the CCL discussion) than MP2 might be better to describe dynamic correlation, why ? In fact, I begun to be suspicious of MP2 schemes, when I realized that most MP2 energy is coming from the highest virtual MOs, which feature extremely bizarre contours ( plot them !) and also very large MO coefficients. 2/ For frequencies computed within the harmonic approximation, scaling factors might correct both for lack of proper correlation treatment,and lack of anharmonicity. It is safer, whenever possible, to try to separate these 2 issues. In fact MP2 frequencies, often seem better than MCSCF frequencies but this might come from a compensation of error with anharmonicity. See my WWW paper : URL : http://yvette.ensta.fr/~muguet/papers/ecc2.html Francis ====================================================================== Francis F. Muguet Ph.D Research Scientist L.O.A Ecole Polytechnique - ENSTA Centre de l'Yvette 91120 PALAISEAU FRANCE Ph: (33) (1) 60.10.03.18 ext. 12.76 Fax: (33) (1) 60.10.60.85 E-mail : muguet@poly.polytechnique.fr *or* muguet@enstay.ensta.fr Web : http://yvette.ensta.fr/~muguet =====================================================================